1. Field of the Invention
The present invention relates to a thin film transistor (TFT) and, more particularly, to a sensor TFT used in an optical detecting sensor that can detect light reflected from an object.
2. Discussion of the Related Art
A thin film transistor type optical detecting sensor can be used as an image reader of an image detecting system such as a character recognition system, a fingerprint recognition system and a telecopy machine.
As shown in FIG. 1A, such a TFT type optical detecting sensor comprises a window through which light is transmitted, a sensor TFT (ST) for forming an optical current using light reflected from an object, a current charging part, or a storage capacitor (SC) for charging current flowing through the sensor TFT (ST), and a switching part, or a switch (SW) for selectively discharging the current charged in the current charging part (SC).
In operation, when light reflected from the object to be read is transmitted to an active area formed between drain and source electrodes of the sensor TFT (ST), an optical current flows along the active area. The optical current is then transmitted to an external circuit through the current charging part (SC) and the switching part (SW). At this point, the optical current corresponds to information on an image of the object. That is, the amount of the optical current varies according to the strength of the reflected light. In addition, the amount of the optical current further depends on the length and width of the active area to which the reflected light is introduced.
For example, when the length of the active area is fixed, the amount of the optical current is increased as the width of the active area is increased.
As is well known, as the amount of the optical current is increased, the image information becomes more accurate. Accordingly, by enlarging the width of the active area relative to the length, the amount of the optical current can be increased. However, when the width is increased, since the sensor TFT occupies much space, it is very difficult to improve the degree of integration of the sensor.
To solve the above problems, a method has been developed for increasing the current ratio as a function of light intensity by reducing an off current flowing along a semiconductor layer of the sensor TFT. To realize this, a second sensor gate electrode is provided between a first sensor gate electrode and a semiconductor layer.
FIG. 1B shows a conventional sensor TFT.
The conventional sensor TFT comprises a first gate electrode 23 for performing an On/Off operation of a transistor by receiving a voltage from a gate wiring; second gate electrodes 27a and 27b disposed on the first gate electrode, the second gate electrodes 27a and 27b spaced away from each other in parallel; a semiconductor layer 31 formed on the second gate electrodes 27a and 27b; and source and drain electrodes 33 and 35 disposed on the second gate electrodes 27a and 27b, respectively.
An exposed portion of the semiconductor layer 31 between the source and drain electrodes 33 and 35 is an active area or conducting channel which has a length L and a width W. That is, the length L becomes a channel length of the semiconductor layer along which the optical current flows, and the width W becomes a channel width of the semiconductor layer.
FIG. 2 is a sectional view taken along line II--II of FIG. 1B for illustrating a manufacturing process of the sensor TFT.
A metal conductive layer is first deposited on a glass substrate 21, then patterned into the first gate electrode 23. A first insulating layer 25 is formed on the substrate, covering the first gate electrode 23.
The second gate electrodes 27a and 27b are formed on the first insulating layer 25, then a second insulating layer 29 is formed on the substrate while covering the second gate electrodes 27a and 27b.
An amorphous silicon layer is deposited on the second insulating layer 29, then patterned into the semiconductor layer 31.
A contact hole 28 is formed on the second insulating layer 29 so that the drain electrode 35 can be electrically connected to the second gate electrode 27a.
Next, the source and drain electrodes 33 and 35 are formed on the second insulating layer 29 while respectively covering both edges of the semiconductor layer 31.
Finally, a protecting layer 37 is formed covering the semiconductor layer 31, and the source and drain electrodes 33 and 35.
In the above described sensor TFT, the first gate electrode 23 is always applied with a negative voltage as the sensor TFT operates with an optical current created by light in an Off state. The optical current created by the light reflected from an object flows along the semiconductor layer 31. At this point, a hole is generated at a portion of the semiconductor layer 31 contacting the second insulating layer 29 by the negative voltage applied to the first gate electrode 23. A current generated by the hole is called an Off current. In this state, when the light is radiated, electron-hole pairs are formed on the semiconductor layer 31 by the light energy.
The holes of the electron-hole pairs are directed to the source electrode 33 along the hole channel formed by a gate voltage, and the electrons are directed to the drain electrode 35 to produce optical current.
Since there is a limit to an amount of the optical current generated in a TFT having a predetermined ratio between the width and length of the channel, if the amount of the off-current is too much, the display quality will be not good.
Accordingly, to increase an optical current ratio by reducing the amount of the off-current, the second gate electrode 27a is provided between the semiconductor layer 31 and the first gate electrode 23. That is, to apply a positive voltage to the source and drain electrodes 33 and 35, the drain electrode 35 is connected to the second gate electrode 27a so that an equi-potential can be generated on the semiconductor layer 31 disposed between the drain electrode 35 and the second gate electrode 27a. The equi-potential characteristic suppresses the generation of holes, reducing the amount of the off-current flowing along the semiconductor layer.
However, in the above-described conventional sensor TFT, since an additional process for forming the second gate electrodes 27a and 27b is further required, the manufacturing process is complicated.